This invention relates to circuitry for automatically and selectively switching a load from one power source, which has failed, to another power source, and automatically reconnecting the load to the first power source when it has regained a predetermined threshold voltage level.
Electrical systems are known in which a battery automatically operates electrical apparatus upon failure of a primary power source normally operating other electrical apparatus, as for instance in a system in which the storage battery automatically operates an auxiliary lighting system when the power serving the main lighting system fails. Examples of such systems are taught in U.S. Pat. Nos. 3,114,095 to Palmer and 3,223,913 to Kalns et al.
Other systems employ a single load normally connected to a primary power source which, upon failure, can be connected to a secondary power source of matched voltage such as a battery which has been maintained in a charged condition. However, these systems do not provide for automatically disconnecting the load from the secondary power source when that source drops in voltage below a predetermined cut-off level. For instance, certain battery systems may suffer irreparable damage if they are discharged below a certain voltage corresponding to a given percentage of capacity, e.g., when 50 to 95% of the available capacity of the battery has been discharged.
It is a primary object of this invention to provide circuitry for atuomatically and selectively switching a load from a primary source to a secondary power source, and also preferably providing for automatically disconnecting the load from the secondary power source when that source has dropped to a predetermined cut-off voltage level as sensed by the circuitry.